Capacitive micromachined ultrasonic transducers (CMUTs) are electromechanical energy conversion devices used to transmit and receive ultrasound. CMUTs used in immersion are generally composed of vacuum-sealed cavities formed by a membrane material. The vacuum-sealed cavities are conventionally realized by two techniques. First one is the sacrificial release process where sacrificial material deposited before the membrane material is etched through the etch holes, and etch holes are filled by deposition under low pressure to form the cavity. CMUTs fabricated by sacrificial release process mostly feature Si3N4 membranes. The limitations of sacrificial release process to achieve very large membranes without breaking or very small membranes with high fill factor adversely affect the precise engineering of the transducer physical parameters. The Si3N4 membranes are also hard to present well-controlled deflection profiles due to the process-dependent residual stress in the membrane material. The second one is direct wafer bonding method where two wafers (one having cavity patterning and the other having the membrane material) are bonded under elevated temperatures under vacuum.
Among these microfabrication methods, direct wafer bonding technology is more economical offering better process control, higher yield, and more novelties in CMUT designs than the sacrificial release process. Direct wafer bonding technology enabled development of single crystal silicon membrane CMUTs rather than silicon nitride membrane ones. As the membrane and the substrate material are both silicon, direct wafer bonding at high temperatures (1100° C.) is achieved without introducing any residual stress in the membrane. Furthermore, IC compatible direct wafer bonding at lower temperatures (400° C.) can be also utilized. This technology has significantly reduced the complexity and the time of the processing of CMUTs additionally offering superior process control, high yield, and improved uniformity compared to the already mature sacrificial release process. Best part of wafer bonding technology is to present a well-known silicon crystal material as a membrane, and to achieve vacuum sealed cavities without the need to open etch holes on the membrane, both of which directly translate into reliable operation in immersion. Recently, successful flip-chip bonding of IC and wafer-bonded 2-D CMUT arrays incorporating through wafer trench-isolated interconnects has been demonstrated. Therefore, recent developments enable the specifications of the CMUT design be comfortably satisfied facilitating the realization of industrial grade CMUT products using direct wafer bonding technology.
Energy conversion efficiency of CMUTs has been of primary importance for ultrasound applications, and improvement of this efficiency has been extensively studied for ultrasound transducers. Conventionally, the CMUT is biased at a voltage below the collapse voltage, and an AC signal is applied to generate ultrasound. The efficiency of the transducer is drastically improved as the bias voltage approaches the close vicinity of the collapse voltage. However, this high efficiency comes with a risk of membrane collapse onto the substrate. Additionally, the AC amplitude is limited to a small excitation voltage around a large bias voltage to prevent membrane collapse during operation. Therefore, the maximum output pressure of a CMUT is inherently limited by the requirements of the conventional operation.
For potential applications such as high intensity focused ultrasound (HIFU) in medical therapeutics, larger output pressures are essential. To offer unprecedented acoustic output pressure in transmit without the aforementioned limitations, novel CMUT operation modes of collapse and collapse-snapback are introduced. Both operation modes require the membrane to contact the substrate surface, which poses a problem on the durability of the membrane in terms of structural integrity and tribological property. Large membrane deflection at collapse increases the stress within the membrane, and change of stress at ultrasound frequencies causes reduced lifetime and compromised reliability in these high output pressure operation modes. Ultrasound applications require the transducer surface to be in contact with the acoustic medium. Because the surface is subject to environmental conditions as well as external pressures, the durability of the membrane defined by hardness is also a major criteria for CMUT performance. Because of electrostatic forces in addition to the atmospheric pressure due to the vacuum sealed cavities, Young's modulus of the membrane plays an important role in the membrane deflection profiles as well.
Collapse-snapback mode requires the collision of the contacting surfaces every cycle, and heat released needs to be dissipated quickly to maintain stable operation. Based on the additional requirements of these modes to reach high output transmit pressure at a sustainable transducer operation, diamond is proposed as the ultimate solution to be used as the membrane material. Mechanical (high Young's modulus, extreme hardness), thermal (large thermal conductivity, low thermal expansion coefficient), and electrical properties (insulator, large electrical breakdown field) of diamond are all in favor of its use in the microfabrication of CMUTs. Chemical inertness, biocompatibility and surface modification are further benefits of diamond for CMUTs to be utilized in corrosive environment and biological samples, respectively. For example, hydrophilic O2-terminated diamond surface, achieved by oxygen plasma or piranha wet processing, will withstand against the detrimental cavitation shock of bubbles in immersion. Because no wet chemical etchant of diamond exists, its use is best suited for extreme and harsh environments. Compared to all potential membrane materials as well as current membrane materials of Si3N4 and silicon, diamond distinguishes itself based on high Young's modulus and exceptional hardness (see Table 1 for material properties of Si3N4, Si, and diamond).
TABLE 1MaterialPropertySilicon NitrideSiliconDiamondYoung's Modulus (GPa)3201601200Hardness (kg/mm2)1580100010000Thermal Conductivity (W/mK)301512000Thermal Expansion (10−6/K)3.32.51.1
Diamond is a perfect membrane material candidate based on its material properties. However, unmature single crystal diamond (SCD) deposition technologies prevented diamond membranes integration into CMUTs. Thin film SCD coated wafers are not commercially available for batch MEMS processes. Surface roughness of SCD is also high to be utilized for CMUT microfabrication based on direct wafer bonding technology.
Recently, with improvements in diamond material growth and technology, ultrananocrystalline diamond (UNCD) as a thin film was made commercially available.
UNCD shares a large portion of the benefits of the SCD with compromised features such as reduced resistivity due to graphitic forms enclosing polycrystalline diamond (SCD: insulator, UNCD: highly resistive). A remarkable feature of UNCD as a membrane material is its deposition as a thin film over a wafer surface with very low residual stress (i.e. <50 MPa). UNCD, featuring smaller grain size and surface roughness has been recently explored for microelectromechanical systems (MEMS) applications such as RF MEMS resonators and hybrid piezoelectric/UNCD cantilevers. However, there are no studies of CMUTs with diamond membranes.
In the documents U.S. Pat. No. 7,846,102B2 and U.S. Pat. No. 7,745,248B2 disclosing various improvements regarding CMUTs, it has been merely mentioned that diamond can be used in the membrane material amongst other materials such as silicon, silicon nitride or silicon carbide.
In the document U.S. Pat. No. 7,530,952B2, a CMUT incorporating direct wafer bonding between the membrane and the substrate is disclosed. It has also been mention in said document that the membrane material can be of diamond amongst other materials such as silicon, silicon nitride or sapphire.
The inventions disclosed in the above mentioned documents, U.S. Pat. No. 7,846,102B2, U.S. Pat. No. 7,745,248B2 and U.S. Pat. No. 7,530,952B2, are not concerned with providing a method to use diamond in the membrane and thus, neither the characteristics of the diamond material to be used nor the means for such use of diamond are not established.
An inconvenience arises in the use of diamond in a membrane to be joined to the substrate by direct wafer bonding, due to the surface properties of diamond layers grown on a substrate as is required for direct wafer bonding. The high surface roughness of such a diamond layer and the low chemical affinity between diamond and silicon dioxide hinders the establishment of the desired direct wafer bond. Moreover, applying conventional polishing methods on a diamond layer does not improve the direct wafer bonding abilities of the diamond layer.